The present invention relates to a control device and control method for an engine having fuel injection valves for directly injecting fuel into combustion chambers.
Recently, vehicular engines of a type that switch the combustion mode in accordance with the running state of the engine have been proposed and put into practical use to improve fuel efficiency and to provide sufficient engine power at the same time. Such engines have fuel injection valves for directly injecting fuel into combustion chambers. In a high engine speed and high load state where high power is required, fuel is combusted while it is evenly mixed with air so that sufficient engine power is provided. Such combustion is called homogeneous charge combustion. To perform homogeneous charge combustion, fuel is injected into a combustion chamber during the suction stroke of the engine. The injected fuel is evenly mixed with air in the combustion chamber and the homogeneous mixture of air land fuel is ignited by an ignition plug.
In a low engine speed and low load state where high power is not required, stratified charge combustion is executed to improve fuel efficiency. In stratified charge combustion, the fuel concentration around the ignition plug is increased to improve ignitability and combustion is carried out while the average air-fuel ratio of the mixture in the combustion chamber is set greater than the stoichiometric air-fuel ratio. To perform stratified charge combustion, fuel is injected into the combustion chamber during the compression stroke. The injected fuel strikes a dent that is provided in the top of the piston and is gathered around the ignition plug. The mixture of the gathered fuel and the air in the combustion chamber is ignited by the ignition plug.
As the combustion mode is switched between homogeneous charge combustion and stratified charge combustion in accordance with the running state of the engine in the manner described above, the fuel efficiency can be improved and sufficient engine power is acquired.
Because such an engine directly injects fuel into combustion chambers, which are under high pressure, it is necessary to set the pressure of fuel to be supplied to the fuel injection valve relatively high. For example, Japanese Unexamined Patent Publication No. 10-176587 discloses an apparatus that supplies fuel under high pressure to fuel injection valves. In this apparatus, a high booster pump pressurizes fuel that has been fed out from a fuel tank by a low booster pump. The pressurized fuel is supplied to the fuel injection valves. Accordingly, the fuel injection valves can directly inject fuel into combustion chambers under high pressure.
When an abnormality occurs in a high-pressure fuel feeding system that includes a high booster pump and a control unit for the pump, the above-described apparatus stops driving the high booster pump and the fuel injection valves inject fuel based on the pressure of the fuel that is delivered from the low booster pump. However, the pressure of the fuel that is fed from the low booster pump is lower than the pressure of the fuel that is pumped from the high booster pump. To acquire the desired amount of fuel injection, therefore, it is necessary to increase the fuel injection time according to the lower fuel pressure.
However, there is a limit to the period during which fuel can be injected in the combustion chambers (injectable period). As the fuel injection time increases, therefore, it becomes difficult to complete the fuel injection within the limited injectable period. When the engine is in a high speed and high load state, particularly, the injectable period is shorter as the engine speed increases, and it is necessary to increase the fuel injection time as the engine load increases. This makes it more difficult to finish fuel injection within the injectable period.
If fuel injection cannot be completed within the injectable period, the amount of fuel in a combustion chamber does not reach the desired value, so that the air-fuel ratio of the mixture gets larger than the adequate value. In this case, the mixture in the combustion chamber is not surely ignited, which may cause misfire. When misfire occurs, unburned mixture enters the exhaust passage of the engine from the combustion chamber. If the unburned mixture is combusted by the heat of the exhaust passage, an exhaust emission purifying catalyst provided in the exhaust passage may be overheated and damaged.
The present invention has been achieved in view of the above-described circumstances and the object of the present invention is to provide a device and method for engine control which can prevent the occurrence of misfire when an abnormality occurs in a fuel supply device.
To achieve the object, the present invention provides a control device for an engine. The engine produces power by burning a mixture of air and fuel in a combustion chamber. The control device includes a fuel injection valve for directly injecting fuel into the combustion chamber and a fuel supply device for supplying fuel to the fuel injection valve. The fuel supply device includes a primary feeding unit and a secondary feeding unit. The secondary feeding unit further pressurizes fuel fed from the primary feeding unit under a predetermined pressure and supplies that fuel to the fuel injection valve. Control means controls the fuel supply device. When an abnormality occurs in the secondary feeding unit, the control means stops driving the secondary feeding unit such that the fuel injection valve performs fuel injection with fuel pressurized by the primary feeding unit. When the secondary feeding unit is stopped, restriction means restricts driving of the fuel injection valve or the amount of air to be drawn into the combustion chamber such that only a mixture that does not cause misfire is formed in the combustion chamber.
The present invention also provides a control method for an engine for acquiring power by burning a mixture of air and fuel in a combustion chamber. The control method includes supplying fuel to a fuel injection valve from a fuel supply device. The fuel supply device includes a primary feeding unit and a secondary feeding unit. The secondary feeding unit further pressurizes fuel fed from the primary feeding unit under a predetermined pressure and supplies that fuel to the fuel injection valve. The control method further includes directly injecting the pressurized fuel into the combustion chamber from the fuel injection valve; stopping the secondary feeding unit such that the fuel injection valve performs fuel injection at the pressure of the fuel fed from the primary feeding unit, when an abnormality occurs in the secondary feeding unit; and forming only a mixture that does not cause misfire in the combustion chamber, when the secondary feeding unit is stopped.
When driving of the secondary feeding unit is stopped, only a mixture that does not cause misfire is formed in the combustion chamber, and a mixture that causes misfire is not formed in the combustion chamber. Therefore, the mixture in the combustion chamber is reliably ignited and no unburned mixture is discharged from the combustion chamber.